Diketones photolysis

The inference that semidione radicals are intermediates in diketone photolysis has been widely accepted for many years since it provides a satisfactory rationale for the results observed in many reactions. Recently, direct evidence for the presence of these radicals has been obtained from flash photolysis 13> of benzil in alcoholic solvents and esr studie of irradiated solutions of biacetyl 12,171,183 camphorquinone 9,i83)> and other diones 183>. The related radical anions, generated by chemical means, have been studied extensively 5,3i,i3 ) and reports of the radical cations have also appeared m.137)... 

Nalidixic acid is stable up to five years under reasonable conditions of temperature and humidity. Pawelczyk and Plotkowiakowa(17) subjected sodium nalidixate solutions to accelerated aging, but were unable to identify decomposition products. Detzer and Huber(lS) studied the photolysis and thermolysis of nalidixic acid in the presence of oxygen. Photolysis produced de-carboxylated nalidixic acid, structure A, and a diketone product, structure B, as well as carbon dioxide and ethylamine. 

The Cotton effects in mixed amino acidate/acetylacetonate complexes [Cr(acac)2L] (L = l-alanine, L-valine or L-phenylalanine) have been studied 774 absolute configurations were assigned by reference to the parent tris(acetylacetonate) complexes. Synthesis was achieved by the photolysis of mixtures of the amino acid and [Cr(acac)3]. The partial photoresolution of both cis- and irans-(l,l,l-trifluoro-2,4-pentanedionato)chromium has been accomplished by irradiation with circularly polarized light (5461 A) in chlorobenzene solution.775 The results indicated that both bond rupture and twist mechanisms were important. A number of other jS-diketonates have also been investigated.776... 

A stereospecific rearrangement is observed in the photolysis of 17j8-hydroxy-4a,5a-epoxy-4(8-methylandrostan-3-one (18) to give the j8-diketone (19)21 the analogous 4/3,5/3-epoxysteroid is converted in the same manner into the diketone with a 5a-methyl group. 

The photolysis of cyclic ketones, in general, results in homolytic cleavage of the carbon-carbon bond adjacent to the carbonyl function. For details of this process, the reader is referred to the review by Srinivasan.425 Included among the photoproducts are certain oxygen heterocycles, formed by cyclization of reactive intermediates. Thus, cleavage in a cyclic /3-diketone, such as tetramethylcyclobutane-1,3-dione or hexamethylcyclohexane-l,3,5-trione [Eq. (110)] is followed by cyclization on the oxygen atom to form a lactone.426,427 y-Lactone formation is also observed on photolysis of 2,2-diphenylindane-l,3-dione [Eq. (Ill)] in ether, and the process is reversible.428... 

Upon direct photolysis, the a-(l-hydroxyalkyl)-substituted a, 3-unsaturated ketones 104 gave 1,4-diketones 106 in moderate yields (Scheme 4.47) [71], with the reaction involving an intermolecular 3-hydrogen abstraction. The resultant vinyli-denecydopropanols 105 underwent a formal double tautomerization to afford 106. 

An interesting photochemical approach to 4,5-disubstituted N-alkylimidazoles consists of the photolysis of 2,3-dihydro-5,6-disubstituted-pyrazines that can be easily prepared from 1,2-diketones and 1,2-diamino-alkanes. For example, the preparative-scale photolysis, in absolute EtOH with high-pressure Hg lamp (Pyrex filter), of 5,6-dimethyl- or 5,6-diphenyl-2,3-dihydropyrazines 54, yields the corresponding N-methyl-imidazoles 57 in high yields (Scheme 12.16). The reaction mechanism involves the formation of an enediimine intermediate 55, followed by cyclization and re-aromatization [41]. 

Berberines are reported to be reduced to dihydroberberines rather than the tetrahydro-compounds by sodium bis(2-methoxyethoxy)aluminium hydride.80 Partial reduction of coralynium salts affords the tertiary dienamine (55), autoxida-tion of which, at pH 8, yields the betaine (56) this can be oxidized by peracids to 6 -acetylpapaveraldine (57), obtainable directly from the dienamine (55) by photolysis in the presence of air. The diketone reacts with hydrazine to give the 1,2-diazine (58).81... 

In hydrocarbon solvents, /3-diketones are predominantly (> 90%) enolized and these solutions have been subjected to flash photolysis, which causes photoisomerization to the diketo form9). Reversion of the diketone to the more stable cis enol was then followed by UV spectroscopy and pseudo first order rate constants at room temperature were in the range 14-68 x 10-3 s-1 (e.g. AA = 23 x 10-3 s 1) with half-lives of several hours. The same research also reveals an alternative transformation on irradiation. In this the cis enol form is converted by rotation about a C-C or C=C bond into one of the possible trans enol isomers. These may then go on to the diketo form but mainly they revert very rapidly to the cis enol with rate constants of 0.1 to 70 s 1 (e.g. AA = 0.27 s-l)9). 

The electrophilic PhCOCH2 radical generated by photolysis of PhCOCH2HgCl in DMSO adds readily to enamine 10b to form the substituted enamine 18, which upon hydrolysis gives the 1,4-diketone 19 in 60% overall yield29. In this free radical chain reaction (Scheme 2), the electron transfer from the easily oxidizable adduct radical 17 to PhCOCH2HgCl is facilitated by the fact that the irreversible half-wave reduction potentials of alkylmercury halides are typically more positive than — 0.6 V30. 

A most expeditious synthesis of triquinacene was described by Deslongchamps and his co-workers in 1973 (Scheme 58).3S°) This approach comprised degradation of Thiele s acid (363) to diketone 364 and photolysis of this intermediate to give keto aldehyde 365. Aldolization of365 led to construction of the tricyclic nucleus sequential hydride reduction, mesylation, and elimination of this intermediate over alumina efficiently provided 356. 

Thermolysis or photolysis of 68 resulted in the formation of mainly the 1,6-diketone 69 and elemental selenium only a trace of 68 was detected by 111 NMR spectroscopy in the photolysis (Scheme 2) <2000JOC1799>. 

Pyrolysis, photolysis, and electronolysis of ethylene oxide (128) yielded similar products (P. Brown et al., 1966). Electron impact-induced fragmentation (129) of the epoxide (51) (P. Brown et al., 1966) was similar to the photochemical behaviour (130) of the epoxide (52) (Kristinsson and Griffin, 1966). Rearrangement of the a,/3-epoxy-ketone to the diketone after ionization (131) has been suggested to account for... 

The contrast between thermal and photochemical reactions of vinyl cyclobutane-dione 36 is of interest60>. Photolysis of 36 proceeded quantitatively with bisdecarbon-ylation to tetrachlorodiene 37 while thermolysis (70°) of 36 resulted in quantitative isomerization to isomeric diketone 39 (the photochemical precursor of 36). Similar effects have been observed with related compounds. The most reasonable mechanism for isomerization 36 - 39 is homolysis to biradical 38 which can either revert to 36 or... 

Another example is the synthesis 73) of tetrachloro Dewar anthracene 100 via diketone 99. Photolysis of 99 produced the desired skeleton which was transformed into 100 by conventional methods. 

Removing the double bond one carbon further from the dione results in the presence of an allylic hydrogen atom which is suitably disposed for abstraction. This was indeed observed with 156a, b when photolysis was performed at temperatures below 5°. The vinyl cyclobutanolones (157, mixtures of stereo-isomers) formed were thermally unstable and isomerized to diketones 158. When irradiations were performed without cooling, the products observed were bicyclic ketooxetanes 159 as expected by analogy with 153. When abstractable hydrogens were not available as in 156c, inefficient formation of bicyclic ketooxetane 160 was observed. 

In fact, as has been noted earlier 2), no clear-cut proof exists as yet for the occurrence of a-cleavage in photolysis of diketones with the exception of the special case of four-membered ring diones. 

In the case of trivalent transition metal diketonates, ML3, such as manganese, iron or cobalt, photolysis at the LMCT band in hydrogen-atom-donating solvents (511) leads to the formation of a ML2 derivative and /3-diketone, the quantum yield depending on the excitation wavelength and the solvent. The mechanism reported in equations 68a-c has been postulated, where [ML2L] represents the species which formally contains the M(II) cation bound to the /3-diketonato radical [L]. ... 

The -diketones 65—68 all eliminated CgOg on photolysis to give the expected product. Thermolysis results indicated elimination of C2O2 however, 65 and 68 gave tars instead of the expected hydrocarbons. The mass spectra of 65—68 showed intense ions for loss of C2O2 and low intensity (M—CO)+ ions. o )... 

Levopimaric acid forms an epidioxide (63) which thermally rearranges to a di oxide (65). On photolysis the epidioxide gives both the diepoxide and the 12-keto-epoxide (64). Treatment of the epidioxide with triphenylphosphine affords a monoepoxide which was shown to be the 8a(14a)-epoxide (66). On further epoxidation this gave the diepoxide, levopimaric acid dioxide. The photochemistry of ring c diterpenoid y-hydroxydienones has also been investi-gated. " Thus compound (67 R = H or Et) afforded firstly the spiro-diketone (68) and then the unsaturated y-lactone (69). 

Dioxane ring contraction via Wolff rearrangement is the basis of the synthesis of dioxolane 565 by photolysis of diazo diketone 566 in methanol (85KGS321). 

Treatment of various cyclopropyl derivatives of the endoperoxide of 2,3-dioxabicyclo[2.2.2]oct-5-ene with triethylamine in dichloromethane resulted in an intramolecular redox reaction involving the peroxide moiety to give cyclopropyl hydroxyalkyl ketones. The yields were good dispiro[cyclopropane-l,5 -[2,3]dioxabicyclo[2.2.2]oct-7-ene-6, l"-cyclo-propane] (13) gave 10-hydroxydispiro[2.0.2.4]dec-8-en-7-one (14) in 80 /o yield. Treatment of similar unsaturated endoperoxides with alumina gave diketones, whereas thermolysis and photolysis of 13, like other unsaturated endoperoxides, afforded an mixture of an... 

Diketones.- A detailed study, by product analysis and by laser flash photolysis, of the photochemical reactivity of the barrelenes (289) has been reported. ... 

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